Incidence of influenza-like illness and severe acute respiratory infection during three influenza seasons in Bangladesh, 2008–2010

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Abstract
To determine how much influenza contributes to severe acute respiratory illness (SARI), a leading cause of death in children, among people of all ages in Bangladesh. Physicians obtained nasal and throat swabs to test for influenza virus from patients who were hospitalized within 7 days of the onset of severe acute respiratory infection (SARI) or who consulted as outpatients for influenza-like illness (ILI). A community health care utilization survey was conducted to determine the proportion of hospital catchment area residents who sought care at study hospitals and calculate the incidence of influenza using this denominator. The estimated incidence of SARI associated with influenza in children < 5 years old was 6.7 (95% confidence interval, CI: 0-18.3); 4.4 (95% CI: 0-13.4) and 6.5 per 1000 person-years (95% CI: 0-8.3/1000) during the 2008, 2009 and 2010 influenza seasons, respectively. The incidence of SARI in people aged ≥ 5 years was 1.1 (95% CI: 0.4-2.0) and 1.3 (95% CI: 0.5-2.2) per 10,000 person-years during 2009 and 2010, respectively. The incidence of medically attended, laboratory-confirmed seasonal influenza in outpatients with ILI was 10 (95% CI: 8-14), 6.6 (95% CI: 5-9) and 17 per 100 person-years (95% CI: 13-22) during the 2008, 2009 and 2010 influenza seasons, respectively. Influenza-like illness is a frequent cause of consultation in the outpatient setting in Bangladesh. Children aged less than 5 years are hospitalized for influenza in greater proportions than children in other age groups.